1. Field of the Invention
The present invention generally relates to implantable medical devices such as implantable cardiac pacemakers and implantable cardioverter/defibrillators, and in particular to protection circuits for protecting the electronic circuits and telemetry circuits of such a device.
2. Description of the Prior Art
Implantable medical devices, including cardiac rhythm management devices such as pacemakers and implantable cardioverter/defibrillators, typically have the capability to communicate data with an external programmer via a radio-frequency telemetry link. A clinician may use such an external programmer to program the operating parameters of an implanted medical device. For example, the pacing and other operating characteristics of a pacemaker are typically modified after implantation in this manner. Modern implantable devices also include the capability for bidirectional communication so that information can be transmitted to the programmer from the implanted device. Among the data which may typically be telemetered from an implantable device are various operating parameters and physiological data. The implantable device generates and receives the radio signals by means of an antenna. Today, antennas capable of far-field communications are of increasing interest in implantable medical devices, in order to allow communication over much greater distances than with inductively coupled antennas.
The technology of cardiac pacemakers has developed in sophistication and functionality over the years. In general, cardiac pacemakers are designed to control the heart by correcting or compensating for various heart abnormalities which can be encountered in human patients. For example, cardiac pacemakers may provide therapeutic stimulation to the heart by delivering therapeutic pulses such as pacing, cardioversion or defibrillation pulses. With this increasing sophistication, however, a concomitant increase has occurred in sensitivity of the implantable devices to misoperation due to external influences such as defibrillation, electrocautery, and the like. Such interference or voltage pulses may be received by the antenna, for example, at galvanic exposure against tissue and may be conducted further into the electronic circuits and RF telemetry circuits of the medical device or may be conducted into the circuits, which is undesirable. This may cause the medical device to falsely identify the interference as being of cardiac origin and give rise to, for example, an erroneous output rate, or in the worst case, it may change the state of or destroy components of the circuits, which, in turn, may severely damage the functions of the medical device.
Hence, there is a need of a circuit that in an effective way can protect the internal circuits of an implanted medical device against undesired voltage pulses caused by exposure to e.g. defibrillation and/or electrocautery.